i have the following problem: After a certain time, the physical resistance of Monk and Paladin decreases. As i was unable to slove this problem, i would be glad as someone could help me to find out the reason for this.

There are a number of ways to lose resistance in the game right now: Being attacked by enemies with the Lower Resist effect (they'll say what % they lower resistances by), certain boons from Dracul, etc. There's also a maximum resistance cap at 65%, so even if you stack resistances beyond that, you can't get further benefits.